A type of X-ray explosion found on neutron stars does not occur near black holes, scientists announced at the 207th meeting of the American Astronomical Society.

The lack of explosions is strong evidence for the existence of a black hole event horizon, a theoretical boundary into which matter vanishes and cannot escape.

“By looking at objects that pull in gas, we can infer whether that gas crashes and accumulates onto a hard surface or just quietly vanishes,” said study leader Ron Remillard, an astronomer at MIT’s Kavli Institute. “For the group of suspected black holes we studied, there is a complete absence of X-ray bursts. The gas that would fuel such bursts appears to vanish.”